There are a few things I know already:
1) 6'x4' table (I have two tables rescued from the closure of Attactix, which I had designed and built for the store).
2) Semi-modular; only in the sense that I want to be able to transport and store the boards so I will build it in 3 2'x4' sections. This will also give me the ability to expand if I want to.
3) Buildings will be removable; Right now I'm thinking that each building will be built on its own 'sabot'. If I can keep the sabots down to a couple of different sizes I should be able to re-arrange the buildings to a certain extent and introduce new ones. That will change the look of Calamity. Maybe I'll take it as far as having brick building replacements for when the massive fire sweeps through town (not an uncommon occurrence in western history).
4) A river or stream; mostly to provide a terrain feature below the level of the table surface. This requirement will be the one that really determines the thickness of the foam I'll need to use for base. If the sabots are on 1" foam I'll need at least another 1" layer underneath that, maybe just go for 2" for a 3" thick board.
5) A railroad; Since I have already been acquiring S Scale equipment and track the railroad is definitely in.
6) Bridge; The citizens of Calamity are going to be a frugal bunch so rather than build and maintain a gallows they will be hanging folks from a the railroad bridge as a cheaper expedient.
7) Alignment and locking mechanism of some kind. I don't want the boards to separate during play and I want them to stay lined up correctly. This especially important since a number of the buildings are going to be located across the seams. Right now I'm thinking something simple like pegs to handle this job.
8) Hide the seams. I know that players aren't likely to be disturbed by the seams very much but I'm looking at a lot of seams if I go with the sabot concept. I'll definitely have to work in scenery to hide those as much as possible.
And here are a few concepts that I have been doodling with:
